1. Field of the Invention
The present invention relates to solar cells, photodetectors and imaging devices that incorporate a strained semiconductor and, more specifically, strained silicon.
2. Description of the Related Art
One way to convert energy from light into electricity is to use silicon to capture photons and produce electron-hole pairs. This is important both for solar cells and for image sensors.
The amount of energy captured by a silicon photovoltaic solar cell (simply “solar cell” in this document) is a function of the wavelengths of the incident light. Silicon has a high absorption rate for energy in the short wavelength portion of the visible spectrum and for UV wavelengths. The absorption rate declines as the wavelengths get longer, and drops to near zero for long wavelength infrared light.
The absorption coefficient of silicon for various wavelengths is shown in FIG. 1. The visible spectrum is the range of wavelengths from about 0.4 microns to 0.7 microns. The absorption coefficient varies by more than by a factor of ten over the visible light spectrum. The band gap of crystalline silicon is 1.2 eV at room temperature. Silicon is substantially transparent to light of photon wavelengths less than its band gap. In contemporary silicon based image sensors, efficiency of detection of red light (photons just above the band gap) is low compared to the efficiency of detection of green and blue wavelengths.
It is known that the band gap of silicon varies when silicon is strained so that its crystal lattice spacing is increased or decreased. If the silicon is under tensile strain so that its lattice spacing is increased compared with unstrained silicon, the band gap is smaller.